Characteristics, Accuracy and Reverification of Robotized Articulated Arm CMMs
Carson Rooms 1 and 2, enter at Carson 1
Mr. R.J. Sanders - Representing University of Warwick, UK
VDI/VDE article 2617 [1] specifies characteristics to describe the accuracy of articulated arm coordinate measuring machines (AACMMs) and outlines procedures for checking them. However the VDI prescription was written with a former generation of machines in mind: manual arms exploiting traditional touch probe technologies. Recent advances in metrology have given rise to non-contact laser scanning tools and robotic automation of articulated arms – technologies which are not adequately characterised using the VDI specification.
In this paper we examine the “guidelines” [1] presented in VDI 2617, finding many of them to be ambiguous and open to interpretation, with some tests appearing even to be optional. The engineer is left significant flexibility in the execution of the test procedures and the manufacturer is free to specify many of the test parameters. Such flexibility renders the VDI tests of limited value and the results can be misleading. We illustrate, with examples using the Nikon-Metris RCA, how a liberal interpretation of the VDI guidelines can significantly improve accuracy characterisation and suggest ways in which to mitigate this problem.
It seems somewhat illogical that an industrial standard in the field of metrology should be underpinned by a set of guidelines rather than strict, prescriptive rules. As such we propose a series of stringent tests and revised definitions, in the same vein as VDI 2617, to clarify the accuracy characterisation process.
The revised methodology includes modified acceptance and reverification tests which aim to include emerging technologies, laser scanning devices in particular, while maintaining the spirit of the existing and established standards. We seek to supply robust re-definitions for the accepted terms “zero point” and “useful arm length”, pre-supposing nothing about the geometry of the measuring device. Finally we propose a novel approach to the testing of probing error based on statistical uncertainty.
[1] Accuracy of coordinate measuring machines; characteristics and their reverification; acceptance and reverification tests for articulated arm coordinate measuring machines.
VDI/VDE 2617, June 2009
